The Open Door Web Site
Part X : The Environment
Environmental Factors Appropriate for Life
THE WATER CYCLE
1. Water evaporates from huge reservoirs known as the seas and oceans. This water evaporates because of the heat from the sun. The water which is left behind is `salty' because all the dissolved substances in the water, called mineral salts, are left behind. This means that the water that is evaporated is purified water.
2. Looking out over the sea, it is impossible to see drops of water moving upwards into the sky. This is because the water which is being evaporated is turned from a liquid form into another form: a gaseous form called water vapor. It is this water vapor that makes the air humid. Sometimes it is possible to feel that the air is humid, but it is impossible to see that water is carried in the air. Water may, of course evaporate in the same way from water sources other than the oceans such as lakes or rivers.
3. As the air carrying the water vapor moves higher and higher, the air becomes cooler and cooler. When the temperature of the air drops to a certain temperature, the water vapor starts to condense into liquid water again in the form of little droplets. When there is enough condensation, it is possible to see the water droplets because they come together as clouds. The more evaporation there is, the more water vapor condenses and so the clouds become bigger and bigger.
Various things may now happen to the water as it reaches the Earth's surface in the form of either rain or snow. If it falls as snow high up on a mountaintop, the snow may stay there over the winter. In the spring, with the spring melt, some of this snow may melt. The melt waters will form mountain rivers, which will run downwards joining other rivers, and eventually flow back into the sea.
Some of the rain falls onto the land where it will form surface water running into rivers that once again will eventually flow into the sea. As the water travels over the surface, mineral salts from the riverbed dissolve in the water.
Some of the rain may filter into the soil and drain downwards. There are holes between the particles of soil where water can accumulate.
Water that is under the surface of the Earth is America. called groundwater. As the water travels through the soil and rocks, minerals dissolve in it. This results in mineral water, such as Evian or Volvic.
Even though these waters now carry dissolved minerals, they are still called fresh water. There are hardly any salts in it compared to the salts in sea water. The rivers enter the sea again replacing the water that was evaporated.
And the cycle starts again...
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